The invention concerns a method of transmitting data signals via an optical fiber between two network elements in a digital data transmission network, wherein, in addition to said data signals, first control and/or monitor signals are transmitted for the administration of the network, and wherein the optical fiber amplifier is inserted in the optical fiber between the two network elements. The invention moreover concerns an amplifier unit for use in such a data transmission network.
Today, optical fibers are extensively used in teletransmission networks. However, a drawback is that attenuation puts a limit to the length of the fibers that can be used. This circumstance has been improved considerably by the development of optical fiber amplifiers, which are purely optical components that are introduced into a fiber, and perform optical amplification of the light which passes through it. Very long fiber lengths can be obtained hereby, as fiber amplifiers, so to speak, replace traditional electrical regenerators or repeaters.
Traditional regenerators in a transmission network, i.e., regenerators where the optical signals are converted into electrical signals, and are amplified and converted again into optical signals, and are usually monitored by transferring a plurality of monitoring signals between each regenerator and its adjacent network elements in the network. Owing to operational reliability, it is desired to monitor the optical fiber amplifiers for the same reason in the same manner.
However, in most transmission systems, such as e.g., SDH or SONET, the monitoring signals are transferred in so-called overhead bytes which are embedded among the other data in the data blocks of the transmission protocols. The signals are therefore accessible only when the data blocks xe2x80x9care packed outxe2x80x9d, as is done, or at any rate may be done, in the traditional electrical regenerators. This, however, is not possible in an optical fiber amplifier, since this merely optically amplifies the light passing through it and therefore has no access to individual bytes in the data flow.
Systems are known wherein separate and special monitoring signals can be transmitted via the fiber or in another manner to or from such optical fiber amplifiers. However, these special signals require special equipment at the fiber amplifiers, as well as at the adjacent network elements. Further, these signals cannot readily be incorporated in the general monitoring system of the network, which, as mentioned, can take place e.g., by means of the overhead bytes of the transmission system. The monitoring of the optical fiber amplifiers will therefore be rather complex.
FR 2 712 096 discloses a system in which control signals can be modulated to the output signal of an optical amplifier. However, monitoring signals proper are not involved.
A system of the type described above with separate and special monitoring signals via the fiber is disclosed in U.S. Pat. No. 5,383,046. The monitoring signals are amplitude modulated on the data signals passing the fiber amplifier. These signals do not have a format allowing them to be incorporated in the general monitoring system of the network because they are transmitted as special signals to the subsequent regenerator proper and only then incorporated into the SDH system.
Accordingly, an object of the invention is to provide a method and an amplifier unit enabling relatively simple monitoring of such fiber amplifiers, and allowing said monitoring to be incorporated directly in the general monitoring system of the network.
This is achieved according to the invention by a method wherein said control and/or monitor signals are transferred between the optical fiber amplifier and at least one of the two network elements in a format corresponding to said overhead signals.
When signals of this format are transferred to or from the fiber amplifier, these signals in the adjacent network elements may be directly incorporated in the general monitoring system, It will therefore be possible to monitor the fiber amplifiers in quite the same manner as if they had been traditional electrical regenerators.
It will be expedient that said second control and/or monitor signals are transferred between the optical fiber amplifier and said network element via an optical fiber, and preferably via the optical fiber in which the optical fiber amplifier is inserted.
If said second control and/or monitor signals are transferred without changing said data signals and first control and/or monitor signals, a system is provided wherein the overall monitoring system has access to all signals as they were before the insertion of the fiber amplifier.
An expedient embodiment is achieved when said second control and/or monitor signals are transferred by means of special optical transmitter/receiver units placed at the optical line amplifier and said network element, respectively.
Said second control and/or monitor signals may e.g. be transferred in the fiber as optical signals having a wavelength which is different from the wavelength of said data signals and first control and/or monitor signals. This ensures that the signals do not interfere with the other optical signals in the fiber. It is a simple operation to separate the signals from the other signals on the receiver side.
Alternatively, said second control and/or monitor signals may be transferred in the fiber by modulation of a carrier wave, which is also used for the transfer of said data signals and first control and/or monitor signals. This method requires a little more complex equipment; but, on the other hand, it does not occupy additional bandwidth in the fiber, which is of importance particularly in networks of much traffic where it is desired to have a bandwidth as large as possible at disposal for the data traffic proper.
The method may be used particularly in a data transmission network which is designed as a Synchronous Digital Hierarchy (SDH), and said second control and/or monitor signals may then be formed on the basis of one or more of the overhead bytes used in SDH. Preferably, one or more of the overhead bytes marked D1-D12 may be used.
As mentioned, the invention also concerns an amplifier unit having an optical fiber amplifier and adapted to be inserted in an optical fiber between two network elements in a digital data transmission network, as described above.
When the unit is adapted to transmit/receive second control and/or overhead signals in a format corresponding to said overhead signals, then, as mentioned before, the signals in the adjacent network elements may be incorporated directly in the general monitoring system in quite the same manner as if it had been traditional electrical regenerators.
An expedient embodiment of the amplifier unit is adapted to receive and/or transmit said second control and/or monitor signals via the optical fiber in which the optical fiber amplifier is inserted.
Likewise, the amplifier unit may be adapted to be used in a data transmission network designed as a Synchronous Digital Hierarchy (SDH), and to allow the second control and/or monitor signals to be formed on the basis of one or more of the overhead bytes used in SDH. In particular, it may preferably be adapted to use one or more of the overhead bytes marked D1-D12.
When the amplifier unit comprises a local control unit which is adapted to generate said second control and/or monitor signals on the basis of the state of the optical fiber amplifier, and/or to control the optical amplifier on the basis of received said second control and/or monitor signals, it is ensured that the amplifier unit may be viewed externally in quite the same manner as a traditional electrical regenerator.
The amplifier unit may e.g., comprise means to separate light having a specific wavelength from the optical signal received via said fiber, and to convert the separated light into said second control and/or monitor signals. Alternatively, it may comprise means to demodulate signals which are modulated on the optical signal received from said fiber, and to convert these into said second control and/or monitor signals.
Correspondingly, the amplifier unit may e.g., comprise means to convert said second control and/or monitor signals into an optical signal having a wavelength which is different from wavelengths occurring in the optical signals amplified by the optical fiber amplifier, and means to multiplex said optical signal with the amplified optical signals. Alternatively, it may comprise means to modulate said second control and/or monitor signals on the optical signals amplified by the optical fiber amplifier. In the latter case, the modulating means may expediently be adapted to modulate a pump laser current to the optical fiber amplifier.
The advantages of separate wavelength and modulation, respectively, are described above.